Slate has an interesting interactive map on their website that shows soccer migration since 1900. Apparently, someone has compiled a list of transfers between clubs and countries during that period and has made the information available spatially. As a soccer fan and a geography teacher, this appears to be the holy grail of maps at first glance.
There’s value in this from a sports perspective, but also a demographic one as well. It would be interesting to compare migration patterns for soccer players to identify source countries for this class of migrant, while at the same time considering the benefits to the local economies and to quality of life (these effects will likely be different for periphery and core nations). The general trend towards player migration has increased with time, beginning in the 1950’s, but really taking off in the 1970’s. At present, over 10,000 players have migrated in the 2012/2013 season alone, and with such increasing player movement there is little doubt the total transfer fee values also increase annually.
You can go to the link indicated above to check out numbers for individual countries, or view the video below the map to see the progression over time.